The injection of minute gas bubbles into a patient's bloodstream for measuring pressure and flow characteristics of blood are disclosed in, for example, DE 29 46 662 A1 and U.S. Pat. No. 3,640,271. In German patent application DE 29 46 662 A1, the velocity of gas bubbles in the bloodstream is measured using ultrasound techniques and from this measurement, the velocity of the bloodstream is determined.
In U.S. Pat. No. 3,640,271, bubbles are sequentially injected into the bloodstream and the velocity of the bubbles is measured using ultrasound imaging procedures. The velocity of blood flow is determined by calculations using equations disclosed in the patent. The bubbles are formed by agitating a container holding a blood compatible solution, such as a saline solution, and a blood compatible gas. The bubbles are withdrawn from the container using a hypodermic needle having a filter such that only bubbles of less than 0.0025 cm are drawn into the syringe. The collected bubbles are then injected into the bloodstream and flow characteristics are measured using sound wave technology including Doppler and ultrasonic techniques.
U.S. Pat. No. 5,249,579 discloses an apparatus for controlled injection of carbon dioxide gas into an animal's vascular system. The apparatus includes a mechanism for controlling the flow rate at which the gas is delivered so that the flow rate of the gas correlates to the flow rate of the blood in the animal's vascular system.
The injected CO.sub.2 functions as a blood displacement media or a contrast medium.
U.S. Pat. No. 5,264,421 discloses an ultrasound structure having a wire for transmission of high power ultrasonic energy. The ultrasonic energy is applied to a vascular occlusion at a duration and intensity sufficient to substantially break up the occlusion and recanalize the patient's artery.
The entire disclosure of each of the references disclosed in the present specification are incorporated herein by reference.